Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder characterized by progressive ataxia, motor impairment, and bulbar dysfunction. Neuropathological findings include loss of cerebellar Purkinje cells, degeneration of spinocerebellar tracts, and neuronal loss in various cranial nerve nuclei. The disease causes death 10-20 years after onset of symptoms because of respiratory failure. We have identified the SCA1 gene and determined that the disease causing mutation is an expansion of a translated CAG repeat which encodes glutamine. We characterized the SCA1 gene product, ataxin-1, in both normal and disease states using transgenic mice and patient tissues. Through the study of animal models, we demonstrated that mutant ataxin-1 causes SCA1 by a gain function mechanism. We also show that mutant ataxin-1 localizes to a 2 mum nuclear structure in brain tissue from SCA1 patients and transgenic mice. Mutant ataxin-1 alters the distribution of the promyelocytic oncogenic domain in the nucleus and interacts with the cerebellar leucine rich acidic nuclear protein (LANP). Based on these data we propose that SCA1 pathogenesis involves the disruption of key nuclear functions and that the selective neuronal degeneration is mediated by the interaction of ataxin-1 with LANP. The overall goal of this proposal is to test these main hypotheses and to investigate the mechanism of pathogenesis in SCA1. To accomplish this goal we will use genetic, biochemical, and cell biological approaches. To begin with, we will characterize a new mouse model for SCA1 generated by inserting an expanded polyglutamine tract into the endogenous mouse Sca1 gene. This animal model will express mutant ataxin-1 in all the neurons typically affected in SCA1. We will generate and characterize mouse models that either lack or over-express LANP to elucidate the normal function of LANP and its role in SCA1 pathogenesis. We will also identify proteins that interact with LANP using the yeast two-hybrid system to identify the potential pathways and/or cellular functions affected by the interaction of mutant ataxin-1 with LANP. We will carry out detailed characterization of the nuclear structures formed by mutant ataxin-1 to gain insight into their role in the pathogenesis and to identify potential mechanisms by which they disrupt neuronal function. Lastly, we will identify new proteins that are down-stream effectors of the SCA1 mutation by carrying out subtractive cloning using transgenic SCA1 mice and wild-type litter- mates. These studies should enhance our understanding of the pathogenic mechanism in SCA1 and other neurodegenerative disorders caused by polyglutamine expansion.